weighted.mean() yields incorrect result when units are effectively [1]

[dholstius]

Summary

It seems that when the units on x are effectively [1], as with day/week or lb/ton, then stats::weighted.mean() yields the wrong answer.

Reprex

library(units)
#> Warning: package 'units' was built under R version 4.2.3
#> udunits database from /Library/Frameworks/R.framework/Versions/4.2-arm64/Resources/library/units/share/udunits/udunits2.xml

w <- c(1, 1) # keep it simple

wtd_mean <- function (x, w, ...) {
  mu <- sum(x * w, ...) / sum(w, ...)
  set_units(mu, units(x), mode = "character")
}

# just to illustrate that `wtd_mean()` does the right thing
# these are all correct
x <- set_units(c(2, 4), "lb/week")
mean(x)
#> 3 [lb/week]
wtd_mean(x, w)
#> 3 [lb/week]
weighted.mean(x, w)
#> 3 [lb/week]

# here, weighted.mean() gets the wrong answer
# note: 0.429 [unitless] would be equivalent to 3/7 [day/week]
x <- set_units(c(2, 4), "day/week")
mean(x)
#> 3 [d/week]
wtd_mean(x, w)
#> 3 [d/week]
weighted.mean(x, w)
#> 0.4285714 [d/week]

# it's not only units of time
x <- set_units(c(2, 4), "lb/ton")
mean(x)
#> 3 [lb/ton]
wtd_mean(x, w)
#> 3 [lb/ton]
weighted.mean(x, w)
#> 0.0015 [lb/ton]

^{Created on 2024-01-10 with reprex v2.0.2}

Session info

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